In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
In dentistry, in addition to metal-supported restorations, all-ceramic restorations are increasingly used. Frameworks made of metal or ceramics are usually provided with several layers of coating and veneering materials to achieve aesthetically particularly attractive restorations.
In the processing of ceramic frameworks a process has now been established in which, irrespective whether using the layering technique or staining technique, a separate glaze firing takes place at high temperatures. In addition to the achieving an optically attractive gloss, the resulting glaze also has the task of equalizing possible unevennesses of the surface and thus reducing the formation of plaque. However, the application of this glaze represents not only a further working step but also an additional thermal stress placed on the dental restoration.
Many different glass-ceramics are known from the state of the art which are suitable as framework material for dental restorations. In recent years, a heat-pressing technique (e.g., IPS Empress® from Ivoclar Vivadent AG, Schaan, Liechtenstein) has been joined by mechanical processing in CAD/CAM systems for the preparation of such frameworks. In particular the CEREC® system from Sirona has found many users worldwide.
Furthermore, glass-ceramics are known from the state of the art which are present in a first crystallization stage for simple mechanical processing, and are subsequently subjected to a further crystallization, also called ceramization, in order to improve the mechanical properties of the framework of the dental restoration. Examples of such framework materials are mica glass-ceramics or lithium silicate glass-ceramics which are mechanically processed in the metasilicate phase and then form the lithium disilicate phase through a further ceramization having a greater strength. Such glass-ceramics are described in detail in DE 103 36 913.
In recent years different grades of stabilized ZrO2 ceramics have found use in dental laboratories and in dental practices. Depending on the degree of sintering in each case, i.e., partly or completely densified materials, different machine and processing systems have become established on the market.
To achieve an aesthetic appearance corresponding to the natural tooth, such ceramic or metallic frameworks are veneered with several layers of glass-ceramics or glasses and finally sealed with a glaze.
There are, however, a series of starting materials with which it is not necessary to simulate the appearance of the natural tooth by the additional application of several layers. Multi-colored blanks for CAD/CAM processing, such as are marketed by Vita under the name Trilux®, are examples of such materials. It is to be assumed that interest in such starting materials will soon increase strongly because an immense amount of time can be saved in the preparation of dental restorations.
It is also possible to process suitably colored starting materials if the restoration prepared from these is not directly visible, such as inlays or onlays in the side-tooth area. These restorations allow the possibility to finalize dental processing with just a glaze.
However, it has now also been shown that materials are increasingly being used which, for easier workability, are processed by means of CAD/CAM in a state in which the highest strength has not yet been reached. After milling or grinding, a further crystallization is triggered by thermal treatment and thus the mechanical properties are improved. If the customary veneering and layering materials are also used, a separate firing is necessary for the desired glaze as the known veneering and layering materials lack the required stability upon firing. However, these customary materials also lack the required dimensional stability at the high temperatures used for firing. This manifests itself in that these materials begin to flow.